In U.S. Pat. No. 7,417,390; Getz et al. wrote, “Brushless direct current (DC) motors typically include electronic circuitry that energizes and de-energizes electric coils (windings) in the motor in order to make the rotor spin . . . . A typical brushless DC motor . . . is packaged in such a way that only two terminals are accessible: a positive power supply terminal VS and a ground terminal GND (also referred to as a positive and a negative rail, respectively). A third terminal which provides a signal that indicates the speed of the motor is sometimes accessible as well . . . . Another solution involves the use of pulse width modulation (PWM). In a PWM scheme, the power supply to the motor is repetitively turned on and off at a fixed frequency but variable duty cycle. When the power supply signal has a relatively low duty cycle, for example 25 percent (that is, the power supply is on 25 percent of the time and off 75 percent of the time), the motor to turns at a relatively slow speed. Increasing the duty cycle causes the motor to spin faster. Full power is achieved by leaving the power supply signal on at all times, i.e., 100 percent duty cycle . . . .
In prior art PWM control schemes for brushless DC motors, the power supply signal is usually driven at full power (i.e., not pulsed) for a fixed period of time at start-up, typically in the range of a few milliseconds to a few seconds, to allow the motor to come up to full speed. The power supply signal is then pulse width modulated to operate the motor at the required speed. Since different motors have different start up times, the fixed period of start-up time for prior art PWM motor drives is typically made longer than necessary to assure that it will be long enough for the slowest starting motors. This is inefficient and generates unnecessary noise . . . .
[Getz et al. disclosed] a start-up sequence for a PWM control . . . . First, the motor is turned on at full power, i.e., the power supply signal is constantly on (not pulsed). The number of motor poles is determined. This determination can be skipped if the number of poles is already known. [T]he speed of the motor is monitored until it reaches a suitable speed. The motor is then driven with a PWM power supply signal. One method for determining when the motor has reached a suitable speed . . . is to count the number of tachometer edges from a tachometer signal. Since a given motor typically takes a certain number of rotations to come up to speed, this provides a rough approximation of the motor speed. A more sophisticated technique for determining when the motor has reached a suitable speed . . . is to measure the time between tachometer edges. Since the number of poles is known, the motor speed can be accurately calculated based on the time between tachometer edges. [T]his method . . . optimizes the start-up time. That is, the power supply signal is switched from constant-on to PWM operation just as soon as the motor reaches a suitable speed. [T]achometer edge or pulse refers not only an edge or pulse in a position signal from an actual tachometer, but also more generally to anything that signifies events relating to the position of the rotor. Thus, if the current monitoring scheme described above is utilized instead of a Hall-effect tachometer, instants of minimum torque would essentially be considered tachometer edges.
[Once the motor of the brushless DC motor has started] a top trace . . . indicates the physical rotation of the motor where O1 indicates the amount of time the motor takes for a first rotation, O2 is for the second rotation, etc. The second trace indicates the undisturbed tachometer signal which provides position and velocity information. The third trace illustrates the PWM power supply signal driving the motor. A and C indicate on times, whereas B and D indicate off times. Th[is] example . . . is for a six-pole (three phase) motor (i.e., six “on” times per revolution). The bottom trace illustrates the actual tachometer output signal from the motor, taking into account the fact that the power supply signal to the motor is being switched on and off to control the speed. The actual tachometer output signal is used to determine the amount of time it takes the motor to complete one rotation.
The normal on time A1 and normal off time B1 for the first rotation are calculated as follows: O1/P=A1+B1 where P is the number of poles in the motor. The duty cycle determines the relationship between A and B: A1=DC(A1+B1) B1=(1−DC)(A1+B1) where DC is the duty cycle (percentage on time).
During the second rotation (.PHI.2), the PWM power supply signal is turned on during times A1 and off during times B1. At the end of the last “on” time A1, the power supply signal is turned off for a shortened “off” time D2, and then turned on for an indeterminate amount of time until a tachometer edge is detected, and then for an additional amount of time equal to A1. As a result, “on” time C2 is longer than A1. By turning the power supply signal on slightly earlier than needed during the last tachometer cycle, it assures that power to the motor will be switched on before the tachometer edge marking the end of the complete rotation. This assures that the entire PWM power supply signal can be resynchronized at the end of each rotation. The “D” off times should be shorter than the “B” off times by as little as possible while still allowing an adequate margin to accommodate changing rotational speeds. Using D=0.75B has been found to provide reliable results. The resynchronization can be accomplished with suitable position sensing technique such as the current monitoring scheme described above.
The motor speed is controlled by varying the duty cycle DC. After a complete revolution is completed, the duty cycle is updated, and the on and off times for the next revolution are recalculated.
The methods described herein can be used with brushless DC motors having any number of poles, and not all poles need be utilized. That is, the motor can be driven by using fewer than all of the poles. For example, . . . the motor can be driven using only phase a and leaving phase b off . . . . This can be helpful in applications where high resolution is required at the low end of the operating range.
The use of Hall-effect transistors to activate and monitor the rotation of a motor is ineffective for numerous reasons. A principal reason is a hall-effect transistor has no conversion from alternating current to direct current. That lack of conversion inhibits the possibility of maintaining a constant high voltage in the armature windings.
The current invention uses a different and more efficient method to turn on and off currents on a brushless motor.